I have this planet in my story that is similar to Earth in many ways. It is inhabited by humans, the flora and fauna are similar, the atmosphere is similar... except not quite.
On Earth, the atmosphere consists of roughly 78% nitrogen, 20% oxygen and 1% other stuff. However, on this other planet, nitrogen takes up a lot less of the atmosphere, coming up in second place with about 30%, with the remaining ~38% being composed of a fictional gas (let's call this Element X) that is about as harmful to living creatures as oxygen and nitrogen are (in other words, completely harmless).
As Element X is a part of the atmosphere, it is also included in the natural production/consumption cycle that, on Earth, simply put, consists of trees converting nitrogen into oxygen and everyone else doing the opposite.
So ignoring Element X, my question is: what would be the effects of the decreased amount of nitrogen? Specifically, I'm referring to the effects it would have on the environment and on the creatures living on the planet, keeping in mind both the planet and its flora/fauna are similar to Earth.